1. Field of the Invention
This invention relates to an image forming apparatus for forming images by image-forming units of multiple colors in an overlapping manner on an image carrier.
2. Description of the Related Art
In an image forming apparatus for forming images by image-forming units of multiple colors in an overlapping manner on an image carrier, it has been known in the past to perform position alignment correction for adjusting image-forming timing with the image-forming units of the colors so that the images of respective colors can be overlaid on one another accurately.
In this method of correction, for example, a method is known in which the image-forming unit of each color is caused to form an image of a predetermined pattern for position alignment, and presence/absence of deviation from an appropriate position is found according to the distance between the patterns.
Japanese Laid-open Patent Publication No. 2001-92202 describes that, in a full-color image forming apparatus having image-forming units of four-color of YMCK, when the full-color image forming apparatus has a single-color mode and a full-color mode, and when it becomes necessary to perform position alignment correction, the correction is made after changing into the full-color mode.
With the method described in Japanese Laid-open Patent Publication No. 2001-92202, the position alignment correction can be performed reliably without relying on the selected mode, but the toners of all the colors are consumed, and in addition, the length of the position alignment pattern is also long, and therefore, there is a problem in that it takes much time in the correction.
There is a need to solve such problem and reduce the image forming material such as the toner and the time required in the position alignment of the images of multiple colors.